1. Field of the Invention
The present invention relates to cable bolts and in particular to tension assemblies for cable bolts suitable for use in the mining and tunnelling industry to provide rock and wall support. The invention is suitable for use in hard rock applications as well as in softer strata, such as that often found in coal mines, and it is to be appreciated that the term “rock” as used in the specification is to be given a broad meaning to cover both these applications.
2. Description of Related Art
Roof and wall support is vital in mining and tunnelling operations. Mine and tunnel walls and roofs consist of rock strata, which must be reinforced to prevent the possibility of collapse. Rock bolts, such as rigid shaft rock bolts and flexible cable bolts are widely used for consolidating the rock strata.
In conventional strata support systems, a bore is drilled into the rock by a drill rod, which is then removed and a rock bolt is then installed in the drilled hole and secured in place typically using a resin or cement based grout. The rock bolt is tensioned which allows consolidation of the adjacent strata by placing that strata in compression.
To allow the rock bolt to be tensioned, the end of the bolt may be anchored mechanically to the rock formation by engagement of an expansion assembly on the end of bolt with the rock formation. Alternatively, the bolt may be adhesively bonded to the rock formation with a resin bonding material inserted into the bore hole. Alternatively, a combination of mechanical anchoring and resin bonding can be employed by using both an expansion assembly and resin bonding material.
When resin bonding material is used, it penetrates the surrounding rock formation to adhesively unite the rock strata and to hold firmly the rock bolt within the bore hole. Resin is typically inserted into the bore hole in the form of a two component plastic cartridge having one component containing a curable resin composition and another component containing a cuing agent (catalyst). The two component resin cartridge is inserted into the blind end of the bore hole and the mine rock bolt is inserted into the bore hole such that the end of the mine rock bolt ruptures the two component resin cartridge. Upon rotation of the mine rock bolt about its longitudinal axis, the compartments within the resin cartridge are shredded and the components are mixed. The resin mixture fills the annular area between the bore hole wall and the shaft of the mine rock bolt. The mixed resin cures and binds the mine rock bolt to the surrounding rock.
Tension assemblies have been proposed to provide tension along cable bolts, for example, which in turn provides a compressive force on the substrate, usually a mine shaft roof substrate, about the bolt. Such tension assemblies often involve hydraulic means for installation and require the installer to lift the means above chest height to be placed on the cable end exposed from the bore hole. This can lead to safety issues, depending on the mine shaft roof height. In one such assembly, with the resin set about the cable portion in the bore hole, a nut placed onto a thread cut into a portion of the outer wires of the cable bolt remaining outside the bore hole. The nut is then rotated on the cable bolt toward and to abut the substrate about the bore hole either directly or through a bearer plate disposed on the shaft between the substrate and the nut. Rotation of the nut is continued for a predetermined number of turns to provide tension along the cable. This method has been found to be unreliable in practice, with failures occurring between the nut and cable. In another method, a threaded rod is coupled onto a distal end of the cable using an external coupling. The coupling is disposed within the bore and the threaded rod is arranged to project from the bore. A plate is then disposed on the rod and a nut threadedly engaged with the rod to capture the plate. The nut is rotated on the rod such that the plate is forced onto the substrate about the bore hole. For this method to work, a portion of the bore hole, adjacent the bore hole opening, must be widened to accommodate the external coupling. This is disadvantageous in that it requires two drilling events when forming the bore hole. If the bore hole is drilled to have one diameter large enough to accommodate the fitting, a larger space is created between the bore hole wall and the cable bolt, requiring more resin to fix the cable bolt in the bore. This has been shown to reduce bond strength between the cable, resin and bore hole wall.